Avoid read barriers for ArtMethod::GetDexFile

Shows up in pmd benchmark from Class::FindDeclaredDirectMethod and
Class::FindDeclaredVirtualMethod. There are still calls to
IsProxyMethod that could probably be eliminated.

ReadBarrier::Mark goes from 12.39% to 3.45% according to perf.

Test: test-art-host

Change-Id: I6a4f2fa2d68bf5f393f83b9b70e8d6fcc9dbdaa2
diff --git a/runtime/art_method.h b/runtime/art_method.h
index 17f343d..7217ead 100644
--- a/runtime/art_method.h
+++ b/runtime/art_method.h
@@ -563,6 +563,7 @@
 
   mirror::ClassLoader* GetClassLoader() REQUIRES_SHARED(Locks::mutator_lock_);
 
+  template <ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
   mirror::DexCache* GetDexCache() REQUIRES_SHARED(Locks::mutator_lock_);
   mirror::DexCache* GetObsoleteDexCache() REQUIRES_SHARED(Locks::mutator_lock_);